As a system for magnifying a viewing angle of a liquid crystal display, there has been known a method in which lights from backlight are collimated, and only lights in the vicinity of the front, good in contrast and tone are extracted and diffused to thereby obtain a display with the same quality as in the vicinity of the front even when viewed at any angle (see, for example, publications of JP-A Nos. 10-333147 and 10-25528).
In a liquid crystal display of this kind, however, a backlight technique to obtain collimated lights is difficult. In a system proposed in the above patent literatures and others, for example, there have been many problems in practical aspects since a backlight system is thick, poor in light utilization efficiency and high in cost.
In an ordinary TN type liquid crystal display without a viewing angle compensating film, a region in which a high contrast can be acquired is only on the order within ±20° of the front. In a case of an STN liquid crystal, the region is rendered narrower. In order to extract only lights in the vicinity of the front, good in display quality, the following two methods are exemplified;
1) a method in which parallelism of backlight emitted lights are confined within a range of the order of ±20° as a half value width and transmitted light in the vicinity of the front is extended with a diffusion means after transmission of a liquid crystal cell to thereby magnify a viewing angle, and
2) a method in which only lights in the vicinity of the front within ±20° are extracted from lights after transmission of a liquid crystal display and spread with a diffusion means.
In the second method, however, a light loss is large and therefore, the method has not been suited for use in liquid crystal display. On the other hand, in the first method, the parallelism is limited within a range of the order of ±40° if a prism light condensing sheet represented by BEF manufactured by 3M Corp. is employed in a back light. And by means of a shape of a backlight guide member is limited within a range of the order of ±40°, which is short in ability for use in a viewing angle magnification system of a liquid crystal display.
As a collimating means, there has been available a method employing a shielding louver represented by a light control film manufactured by 3M Corp. and others. In the method, however, there has been a problem in brightness because of a large absorption loss in the course of collimation. That is, one of a thickness, brightness and a parallelism of obtained lights have to be sacrificed due to a requirement from design, resulting in many of problems for putting it into practical use. Especially, for use in a note book personal computer or a cellular phone, it is desirable to use a collimation system restricted in increase in thickness to 200 μm or less and preferably, 100 μm or less, and even in a case where a collimation system is built in simultaneously together with a reflecting polarizer for adding a brightness enhancement effect, it is desirable to restrict the maximum increase in thickness to 500 μm or less, which is difficult being realized in the method.
On the other hand, there has been known collimation means with a mirror, a lens, a prism or a light guide member. In these methods, however, a thickness and a weight increase greatly, which negates to establish a position as a useful means in applications other than a projector or the like.
Accordingly, in a viewing angle liquid crystal display, a necessity has arisen that not only is collimation effected in a thin film structure, but a light source is also confined within about ±20°, which is a range in which a good viewing angle characteristic of the liquid crystal display can be attained, and that an absorption loss is further reduced.
In a collimation means using a shielding louver, a microlens array, a prism or the like, a moiré occurs between a fine structure and pixels of the liquid crystal display, thereby having disabled a good display to be obtained. Since light is not emitted from a binding portion in a prism, a clearance between lenses or the like, a regular darkness and brightness in-plane pattern arises in emitted light, resulting in a moiré. In order to prevent a moiré, it is possible to insert a diffusion means, whereas a problem arises that a parallelism of obtained collimated lights is degraded, which has been a problem against putting into practical use.
Even in a case where interference between liquid crystal pixels and a collimating means was alleviated by changing a cycle in the regularity, a case was further observed where interference occurs with a fine structure of a collimated light diffusing means arranged on the display surface side of the liquid crystal display. In a case where a structure having regularity such as a microlens array, a microprism or the like is employed in a collimated light diffusing means, interference occurs with the fine structure thereof.
Therefore, in order to prevent interference of liquid crystal pixels with the collimated light diffusing means, a necessity arises for contrivance of a size of the fine structure of the collimated light diffusing means or a way of arrangement thereof. A design for preventing interference with liquid crystal pixels, however, has been easy to cause a problem to lead to second interference between members themselves that would otherwise cause no interference since the design is the same as a means preventing interference of the collimating means with the liquid crystal pixels.
For example, if a collimating means adopts a size of a structure not interfering with the liquid crystal pixels, a collimated light diffusing means similarly adopts a size of a structure not interfering with the liquid crystal pixels; therefore, both means result in having the sizes interfering with each other. This applies to a contrivance of angles and arrangement in a similar way, which has narrowed a range of an allowable design and restricted a selectable optical system within an extremely small range.
In such a way, a viewing angle magnification system consisting of a collimating means and a collimated light diffusing means has limited options in design because of an optical problem caused by respective fine structures, having lead to difficulty putting the system into practical use.
Investigation about collimation for a light source using a special optical film has been conventionally conducted in addition to a type that employs surface structures, refraction and reflection that requires a large depth and an air interface such as a lens and a mirror prism and a front light condensation/collimation system accompanied by a large absorption loss such as a shielding louver.
Typical methods thereof include a method in which a bright-line light source and a band pass filter are combined. Exemplified are: a publication of JP-A No.6-235900 which is filed by Phillips Corp., a publication of JP-A No. 2-158289 a publication of JP-N No. 10-510671, the specification of U.S. Pat. No. 6,307,604, the specification of DE No. 3836955, the specification of DE 4222028 A1, the specification of EP No. 578302 A, the specification of USP No. 2002/34009 A, a pamphlet of WO 02/25687 A1
A method can be exemplified in which a band pass filter is provided on a bright-line emitting light source/display unit such as CRT or electroluminescence, which are described in the specification of JP No. 2001/521643 A and the specification of JP No.2001/516066 A.
Furthermore, exemplified is a method in which a band pass filter adapted for three wavelengths is provided to a bright-line cold cathode fluorescent lamp that is described in the specification of USP No. 2002/36735 A filed for a patent by Fuji Photo Film Co., Ltd. and publications of JP-A Nos. 2002-90535 and 2002-258048 filed by NITTO DENKO CORPORATION; or the like.
The techniques do not function unless a light source has a bright-line spectrum. Therefore, a problem has remained that is related to design and manufacture of a film selectively functioning for a specific wavelength. In addition, in a case where a band pass filter is of an evaporated interference film, even a reliability problem has existed that a wavelength characteristic alters due to a change in refractive index of a thin film in a humidified environment.
On the other hand, as a collimation system employing a hologram material, exemplified are the specification of U.S. Pat. No. 4,984,872 filed by Rockwell Co. and others. A material of this kind is high in a front transmittance, while an obliquely incident light cannot be perfectly reflected and removed off. In a case where a direct transmittance is measured with incident collimated light, a high transmittance is measured in the front direction because the light passes through as is, while a low transmittance is measured for obliquely incident light because the light is scattered, with no difference observed between transmittances in both cases when a diffusion light source is employed. Therefore, in a case where the system is disposed on an actual diffusion backlight light source, a light condensing function cannot be sufficiently satisfied. Many of hologram materials are soft and weak, having lead to many problems related to reliability.
It is an object of the present invention to provide a liquid crystal display of a thin type and capable of realizing a wide viewing angle.